In medical radiography, the image of affected tissue of a patient is formed by recording the pattern of X rays transmitted by the tissue in a photosensitive material which comprises a transparent support having thereon at least one light-sensitive silver halide emulsion layer (i.e., a silver halide photographic material). A transmission pattern of X rays can be recorded by using a silver halide photographic material alone. However, it is undesirable for the human body to be exposed to X rays in quantity, so that a combination of a silver halide photographic material with a radiation intensifying screen is generally used in practicing X-ray photography. The radiation intensifying screen comprises a support having a phosphor layer provided thereon, and the phosphor layer functions so as to convert the X rays absorbed thereby to visible rays to which a silver halide photographic material has high sensitivity. Therefore, the intensifying screen can markedly improve the sensitivity of an X-ray photograph taking system.
For the purpose of further heightening the sensitivity of an X-ray photograph taking system, there was developed the method of using a both-sided emulsion film, or a silver halide photographic material having silver halide light-sensitive emulsion layers on front and back sides of a support respectively, and practicing X-ray photography in a condition such that the film is inserted between two radiation intensifying screens (which may be simply called "intensifying screen"). In ordinary X-ray photography, the above-described photograph taking method is adopted at present. The development of this method originated in that sufficient X-ray absorption was not achieved by the use of only one intensifying screen. More specifically, even if the amount of a phosphor contained in one intensifying screen is increased, the converted visible rays are scattered and reflected inside the phosphor layer since the increased content of the phosphor results in thickening the phosphor layer. Accordingly, the visible rays emitted from the intensifying screen strike divergently on the surface of the photosensitive material disposed in contact with the intensifying screen. In addition, the visible rays generating in the depth of the phosphor layer are hard to get out of the phosphor layer. Thus, the amount of effective visible rays emitted from the intensifying screen cannot be increased even if the thickness of the phosphor layer is increased excessively. On the other hand, the X-ray photograph taking method using two intensifying screens which each contain a phosphor layer having a moderate thickness has an advantage in that the X-ray absorption as a whole can be increased and effectively converted visible rays can be taken out of the intensifying screens.
The research for finding out an X-ray photograph taking system excellent in balance between image quality and photographic speed has so far been carried out continuously. For instance, there has been prevailingly used the combination of a blue light-emitting intensifying screen having a layer containing calcium tungstate as a phosphor with a spectrally unsensitized silver halide photographic material (e.g., the combination of Hi-Screen Standard and RX, both being the products of Fuji Photo Film Co., Ltd.). In recent years, however, the combination of a green light-emitting intensifying screen having a layer containing the terbium-activated oxysulfide of a rare earth element as a phosphor with an orthochromatically sensitized silver halide photographic material (e.g., the combination of Grenex 4 with RXO, both being the products of Fuji Photo Film Co., Ltd.) has come to be used, and has effected improvements in both sensitivity and image quality.
However, a silver halide photographic material provided with photographic emulsion layers on both sides has a problem of tending to suffer deterioration in image quality due to crossover rays. The term "crossover rays" used herein refers to the visible rays which are emitted from each of the intensifying screens arranged on both sides of a photosensitive material, are transmitted by the support (usually having a thickness of 170-180 .mu.m or so) of the photosensitive material and further reach the light-sensitive layer disposed on the opposite side, thereby causing deterioration in image qualities (especially sharpness).
For the purpose of reducing the above-described crossover rays, various arts have so far been developed. For instance, U.S. Pat. Nos. 4,425,425 and 4,425,426 disclose the arts of using spectrally sensitized tabular-grain emulsions having high aspect ratios as light-sensitive silver halide photographic emulsions. According to those inventions, it is possible to reduce the crossover rays to 15-22%. Moreover, U.S. Pat. No. 4,803,150 discloses the art of disposing a layer of a microcrystalline dye capable of being decolored by development-processing between the support and the light-sensitive layer of a silver halide photographic material. Such an invention enables the crossover rays to be reduced to below 10%.
The gradation of photographs (characteristic curve's shape) is very important at the time of making medical diagnoses using photographic images. Recently, it has been diagnosticians' study to properly use photographic materials according to the portion to be diagnosed in human body. Following this trend, photographic film makers provide photosensitive materials various in their characteristic curves. The photosensitive materials on the market today are roughly classified into high contrast photosensitive materials for blood vessel contrasting photography, standard contrast photosensitive material for amateur use, wide latitude photosensitive materials for photographing abdomen and stomach, extremely wide latitude photosensitive materials for photographing thorax, and so on.
Further, various photosensitive materials differing in characteristic curve are disclosed in JP-A-59-214027 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-60-41035, JP-A-60-159741, JP-A-61-116346, JP-A-62-42146, and JP-A-62-42147.
On the other hand, there have been made attempts to find out X-ray photograph taking systems excellent in balance between image quality and photographic speed by combining a silver halide photographic material having photographic emulsions on both sides thereof with radiation intensifying screens under specified conditions. For instance, JP-A-02-266344, JP-A-02-297544 and U.S. Pat. No. 4,803,150 disclose the X-ray photographing systems designed so that the combination of an intensifying screen arranged on the X-ray irradiation side (front intensifying screen) with a light-sensitive layer (front sensitive layer) may be different in spectral characteristic (sensitivity) from the combination of an intensifying screen arranged on the opposite side (back intensifying screen) with a light-sensitive layer (back sensitive layer) and, what is more, the front combination and the back combination may have different contrasts. Further, experimental results of the combinations of the products of 3M Co., Ltd. concerning silver halide photographic materials and radiation intensifying screens are reported in Photographic Science and Engineering, Vol. 26, No. 1, p. 40 (1982). More specifically, the report states that the combination of Trimax 12 (trade name, a commercial intensifying screen of 3M Co.) with XUD (trade name, a commercial silver halide photographic material of 3M Co.) is almost equal in sensitivity and sharpness (MTF) to the combination of Trimax 4 (trade name, a commercial intensifying screen of 3M Co.) with XD (trade name, a commercial silver halide photographic material), but the former combination is higher in NEQ (ratio of noise to output signal) than the latter. Further, the report teaches that the above-described results can be inferred from the fact that XUD shows higher sharpness than XD, while Trimax 12 shows higher X-ray absorption than Trimax 4.
If attention is devoted only to the quality of X-ray images, it goes without saying that high quality images can be obtained by the combined use of a low-sensitivity silver halide photographic material and low-sensitivity radiation intensifying screens. In using a low-sensitivity combination as described above, however, it becomes indispensable to increase an amount of X rays to which human body is exposed (exposure amount). Consequently, such a combination is undesirable for practical use. In the case of a mass examination in particular, wherein most of the subjects are healthy persons, it is impossible to use that combination in practice because it is necessary to strictly avoid an increase in exposure amount.
As described above, there have so far been conducted researches for finding out various types of X-ray photograph taking systems which can provide excellent balance between image quality and photographic speed. However, hitherto developed X-ray image forming methods cannot be yet said to be embodied in X-ray photograph taking systems endowed with image quality and sensitivity high enough for the purpose of thoracic diagnoses using X-ray images. As for the X-ray image of thorax, for instance, it is very important from the diagnostic point of view that the shadow of very thin blood vessels in the lung field can be observed up to their terminal parts. However, conventional X-ray photograph taking systems cannot satisfy such a requirement.
Moreover, another difficulty comes up in photographing the thorax of human body. More specifically, the thorax is photographed using X rays in order to diagnose mainly lungs as a whole, and the X-ray photograph thereof has to depict coextensively a thoracic part by which X rays are transmitted in a relatively large quantity, including middle and upper areas of lungs, and a thoracic part by which X rays are transmitted in an extremely small quantity, including the central shadowy, heart and subphrenic areas. The difference in quantity of the transmitted X-rays, as described in SPIE, vol. 1651 entitled "Medical Imaging VI: Instrumentation" (1992), becomes greater the fatter physique the photographed person has, and it ranges specifically from 0.9 to 1.1 in terms of the logarithm of the difference in exposure amount. Accordingly, it is necessary for a screen/film system to widen its dynamic range (latitude). However, widening the latitude is contradictory to high contrast requirements.
Therefore, only one sheet of photograph taken by a hitherto known method for photographing a thorax with X rays cannot provide wholly satisfactory image information to diagnosticians. That is, the image obtained using a thorax photograph taking system of wide-latitude type, though contrasty in the areas by which X rays were transmitted in a small quantity, such as in the central shadowy and subphrenic areas, is hard to view and to serve diagnosis because the overall impression is one of roughness with the grain coarseness standing out, and the shadow of blood vessels in the lung field is short of contrast and sharpness.
On the other hand, when a photosensitive material of standard gradation is used, the shadow of blood vessels in the lung field has satisfactory contrast, but the image obtained is hard to serve diagnosis because it gives the impression of being one of roughness with the grain coarseness standing out. In addition, the image obtained wants depiction of the central shadowy area and the like, so it is quite inferior in quality. For the purpose of making an improvement in depiction of the central shadowy area and the like, there can be adopted a method of photographing with X rays emitted under a high pressure condition. However, such a method can only provide images having on the whole low contrast and lacking sharpness through the influence of scattered rays. Although an improvement in depiction of the central shadowy area and the like can also be brought about using another method of increasing the exposure amount, the image density in the lung field becomes so high that diagnosis cannot be easily made by the observation with a generally used Schaukasten.